1. Field of the Invention
The present invention relates to an active matrix liquid crystal display panel, and more specifically to a drive system for an active matrix liquid crystal display panel in which an address switching transistor is connected to each of the picture elements for a matrix display pattern.
2. Description of the Prior Art
A thin film transistor (TFT) active matrix liquid crystal display device is known as a typical matrix liquid crystal display device involving nonlinear elements for driving the liquid crystal. This conventional display device incorporates address TFT's arranged in a matrix in the liquid crystal display panel, whereby the display device provides the same high contrast display as achieved by static drive even if it employs multiplex drive with a small duty ratio, i.e. with multiple lines.
The drive system of the TFT active matrix liquid crystal display device may have the circuit construction shown in FIGS. 1 and 3 and signal waveforms shown in FIG. 2. As shown in FIG. 1, a TFT 11c is connected to a liquid crystal display panel 11 at the intersection between a row electrode 11a and a column electrode 11b. A liquid crystal layer capacity is designated by 11d. A row electrode drive 12 is mainly composed of a shift register which shifts and outputs scanning pulses S sequentially to corresponding row electrodes by clocks .phi. sent from a gate signal control 13. When the total scanning period for the row electrodes is represented by T and the number of scanning lines by N, the scanning period H is expressed by the formula: H=T/N. A pulse voltage whose pulse width is equal to the scanning period H is applied to the row electrodes sequentially, thus turning ON the TFT's. A column electrode driver 14 comprises a shiaft register 16, sampling switches, etc. as shown in FIG. 3. The column electrode driver 14 samples data signals transmitted in series from a data signal control 15 and outputs them sequentially to the column electrodes in synchronization with clocks .phi. with timings corresponding to the respective column electrodes, so that the data signals are written in the liquid crystal layer through the TFT's 11c. This drive system, which sample-holds (SH) data signals directly in the display panel, is called a panel SH drive system.
In the panel SH drive system, data sampling and data writing in the liquid crystal layer through TFT's are conducted in the same horizontal scanning period. The period for writing data in the liquid crystal layer, therefore, ranges from 1H or 63.5 .mu.s (one horizontal scanning period) to 11 .mu.s (horizontal retrace period) for a television signal, for instance. Accordingly, the period allowed for writing data in the liquid crystal layer of display picture elements decreases at later sampling timing; the shortest write period is 11 .mu.s.
To effect AC drive of the liquid crystal, the data signal applied for each scanning line reverses its polarity.
With the conventional drive system, as mentioned above, the liquid crystal write period becomes shorter at a later data sampling timing. When the time constant T.sub.ON which is the product of TFT ON resistance (R.sub.ON) and liquid crystal layer capacity (C.sub.LC) is not sufficiently small, the write period is sufficiently long at an earlier sampling timing, so that the liquid crystal layer can be charged through the TFT's with voltage applied to the column electrodes to a specified potential, but the write period decreases at a later sampling timing until the TFT's are finally turned OFF before the liquid crystal has been charged to the specified potential. In such a case, the old data cannot be rewritten completely. The potential applied to the liquid crystal layer is of in-between of the potentials of the old data and the new data. Consequently, data mixed with the old data is displayed on the panel. Thus, a difference in the write period among the display picture elements in the lateral direction of the display panel may result in a display picture of various definition.